This application is based on applications No. H11-086204 filed in Japan on Mar. 29, 1999 and No.11-133782 filed in Japan on May 14, 1999, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a display optical apparatus using a display panel, and a projector display apparatus using the display apparatus.
2. Description of the Prior Art
Conventionally, as a reflective liquid crystal display device used as a display panel, TN (twisted nematic) liquid crystal, homogeneous liquid crystal and DAP (deformation of aligned phases) liquid crystal have been used. These all perform light modulation by use of the birefringent property of liquid crystal. Of these, the TN liquid crystal has a structure in which the liquid crystal molecules are aligned, from the obverse surface to the rear surface of the display panel, horizontally to the surface of the display panel so as to be twisted at a predetermined angle. To cause the TN liquid crystal to act as a reflective display, the above-mentioned birefringent property is used. While the optical rotatory power is used when the TN liquid crystal is used as a transmissive display device, the present invention is not directed thereto.
The birefringent property is a property to cause a phase difference in the incident light by a polarization plane to thereby modulate the polarization condition. In the TN liquid crystal, at each pixel of the liquid crystal, when no voltage is applied, because of the horizontal alignment, a birefringent effect acts on the incident light to cause the modulation, and when a voltage is applied, the liquid crystal molecules are aligned in a direction vertical to the surface of the display panel, so that no birefringent effect acts on the incident light, particularly on the light vertically incident on the surface and consequently, no modulation is caused.
The homogeneous liquid crystal has a structure in which the liquid crystal molecules are aligned horizontally to the surface of the display panel in a predetermined direction. Like the above-described TN liquid crystal, when no voltage is applied, because of the horizontal alignment, a birefringent effect acts on the incident light to cause the modulation, and when a voltage is applied, the liquid crystal molecules are aligned in a direction vertical to the surface of the display panel, so that no birefringent effect acts on the light vertically incident on the surface and consequently, no modulation is caused.
The DAP liquid crystal has a structure in which the liquid crystal molecules are aligned vertically to the surface of the display panel conversely to the homogeneous liquid crystal. When no voltage is applied, because of the vertical alignment, no birefringent effect acts on the light vertically incident on the surface, so that no modulation is caused. When a voltage is applied, the liquid crystal molecules are aligned in a direction horizontal to the surface of the display panel, so that the birefringent effect acts on the incident light to cause the modulation.
In all of these types of liquid crystal, the display of each pixel is turned on and off according to whether the birefringent effect acts on the incident light or not. That is, when the birefringent effect acts on incident light of a specific polarization condition, the incident light is reflected under a condition where it is modulated to a different polarization condition, and when no birefringent effect acts as incident light, the incident light is reflected under a condition where it is not modulated. To obtain a high-contrast image with a display optical apparatus employing a display panel using such a reflective liquid crystal display device particularly when a projector display apparatus is structured, what is important is the level of black display that is, how completely black is displayed, in other words, how much light can be intercepted at the black portion of the displayed image.
Examples of liquid crystals other than the above-described types include ferroelectric liquid crystal (FLC). In this liquid crystal, unlike the above-described types of liquid crystals, the birefringent effect always acts on the incident light, and the modulation is caused by changing the axial direction of birefringence. Compared to the above-described types of liquid crystals, the ferroelectric liquid crystal has characteristics such as a wide viewing angle, memory capability and fast response. FIG. 15 schematically shows the ferroelectric liquid crystal viewed from the front side of the panel.
Assume that, is shown in xcex1 of the figure, the direction of alignment of the liquid crystal molecules m viewed from a direction toward the surface of the display panel p, that is, viewed from the front side is inclined leftward by xcex8 with respect to the broken line 1 representing the reference direction of the figure, for example, when there is no electric field. When an electric field is applied under this condition, as shown in xcex2 of the figure, the direction of alignment of the liquid crystal molecules m is inclined rightward by xcex8 with respect to the broken line 1. In the ferroelectric liquid crystal, the modulation is caused by changing the axial direction of birefringence between these two conditions. Antiferroelectric liquid crystal exhibits substantially similar characteristics optically.
An example of the conventional display optical apparatus is one in which a polarizing plate is disposed immediately in front of a display panel. In this apparatus, when incident light having a specific polarization axis and having passed through the polarizing plate is modulated by the display panel, the incident light is reflected with its polarization axis being rotated 90 degrees and returns to the polarization plate. At the polarizing plate, the incident light is intercepted, thereby providing black display. When the incident light is not modulated by the display panel, the incident light is reflected with its polarization axis being as it is and passes through the polarization plate, thereby providing white display or, in the case of the so-called multi-panel type, display of the color of the display panel.
Another conventional example is one in which a PBS(polarization beam splitter) is disposed immediately in front of a display panel. In this apparatus, of the illuminating light illuminating the display panel, for example, only s-polarized light is reflected at the PBS and the reflected s-polarized light is made incident on the display panel. When the s-polarized light is not modulated at the display panel, it is reflected as it is and returns to the PBS. At this time, the s-polarized light is not transmitted by the PBS (but is reflected toward the side of the illuminating light from which it originates), thereby providing black display. When the incident light is modulated by the display panel, it is converted into p-polarized light and reflected, and the p-polarized light is transmitted by the PBS, thereby providing white display or, in the case of the so-called multi-panel type, display of the color of the display panel. Such modulation is generally called cross nicol modulation.
Yet another conventional example is one in which a quarter-wave plate having an axis of phase retardation or an axis of phase advancement forming an angle of 45 degrees with respect to the axis of polarization of a polarizing plate disposed immediately in front of a display panel is disposed between the display panel and the polarizing plate. In this apparatus, when incident light having a specific polarization axis and having passed through the polarizing plate is not modulated by the display panel, the incident light passes through the quarter-wave plate twice in opposite directions to undergo the working of a half-wave plate, so that the polarization axis rotates 90 degrees. Then, the incident light returns to the polarizing plate and is intercepted at the polarizing plate, thereby providing black display.
When the incident light is modulated by the display panel, the incident light undergoes the working of a half-wave plate equivalent to that which the incident light undergoes when it passes twice in opposite directions through the quarter-wave plate having an axis of phase retardation or an axis of phase advancement forming an angle of 45 degrees with respect to the axis of polarization; and further, the incident light actually passes through the quarter-wave plate itself twice in opposite directions to undergo the working of a half-wave plate. Consequently, the condition of the incident light becomes equivalent to that of the incident light having passed through a one wave plate, so that the direction of the polarization axis returns to the original direction (actually, the axis has been rotated 180 degrees) and the incident light passes through the polarizing plate, thereby providing white display or, in the case of the so-called multi-panel type, display of the color of the display panel.
However, in the above-described conventional structure in which the polarizing plate is disposed immediately in front of the display panel, black display is provided when the incident light is modulated by the display panel, and since the optical phase difference varies significantly according to the wavelength, that is, the color of the incident light under such a condition where the birefringent effect is produced by liquid crystal (wavelength dependence), the polarization axis of the incident light does not always rotate exactly 90 degrees when the incident light is modulated, so that the reflected light from the display panel cannot be completely intercepted by the polarizing plate and it is therefore difficult to provide black display. Moreover, an apparatus is known in which black display is provided by correction with a phase difference film or the like. However, the white-to-black image contrast is only approximately 20:1 in this apparatus.
In the above-described conventional structure in which a PBS prism is disposed immediately in front of the display panel and the so-called cross nicol modulation is performed, although it is alright to provide black display when the incident light is not modulated by the display panel, since PSB prisms are generally expensive and in the case of, for example, so-called three-panel projectors, one PBS prism is necessary for each display panel, the cost is extremely high.
When the ferroelectric liquid crystal is used for the display panel, by making the polarization plane of the illuminating light from the PBS prism and the optic axis of the ferroelectric liquid crystal at the time of black display coincide with each other, excellent black display can be provided irrespective of the above-described wavelength dependence and a phase difference error due to a thickness error of the ferroelectric liquid crystal itself described later. However, although a PBS prism for light of a wide wavelength range from R (red) to B (blue) is necessary when a PBS prism is used for structuring a so-called single-panel projector described later in which full advantage of the fast response of the ferroelectric liquid crystal can be taken, such a wide-wavelength-range PBS prism has so-called angle dependence for the angle of incidence, and is inferior in performance such as the extinction ratio described later.
In the above-described conventional structure where a quarter-wave plate having an axis of phase retardation or an axis of phase advancement forming an angle of 45 degrees with respect to the polarization axis of a polarizing plate disposed immediately in front of a display panel is disposed between the display panel and the polarizing plate, although it is alright to provide black display when the incident light is not modulated by the display panel like in the structure in which a PBS prism is disposed, since the quarter-wave plate also has great wavelength dependence, it is impossible to provide perfect black display for incident light of a predetermined wavelength width. Normally, the polarizing plate and the quarter-wave plate are cemented together and used as one unit. The accuracy of the angle between the axis of the polarizing plate and the axis of the quarter-wave plate at the time of the cementing is one degree at the utmost, so that a white-to-black image contrast of not lower than 50:1 cannot be achieved.
There is an apparatus in which in order to cope with the problem of the wavelength dependence of the quarter-wave plate, two phase plates are cemented together so that the wavelength dependence is eliminated and that an effect the same as that of the quarter-wave plate acts on specific incident light. Although this is not completely equivalent to the quarter-wave plate in actuality and has optical rotatory power as well, the same effect as that of the quarter-wave plate acts on specific polarized light having passed through the polarizing plate. However, because of the cementing error of the polarizing plate and the phase plate similar to that described above, or a slight amount of so-called xe2x80x9cremaining phase effectsxe2x80x9d due to the alignment of the liquid crystal molecules or the angle of incidence of the incident light on the liquid crystal, it is difficult to provide perfect black display.
FIGS. 14A to 14C are cross-sectional views schematically showing conditions of liquid crystal where such xe2x80x9cremaining phase effectsxe2x80x9d is caused. FIG. 14A shows a condition where the liquid crystal molecules m are aligned horizontally to the surface s of the display panel p. In this condition, the birefringent effect naturally acts on the incident light to cause the modulation. FIG. 14B shows a condition where the liquid crystal molecules m are aligned vertically to the surface s of the display panel p. In this condition, although no birefringent effect acts on the light vertically incident on the surface s as shown by the arrow A, a slight birefringent effect acts on the light obliquely incident on the surface s as shown by the arrow B.
In the homogeneous liquid crystal described in the prior art, the liquid crystal molecules are horizontally aligned as shown in FIG. 14A when no voltage is applied, and are vertically aligned as shown in FIG. 14B when a voltage is applied. Conversely, in the DAP liquid crystal, the liquid crystal molecules are vertically aligned as shown in FIG. 14B when no voltage is applied, and are horizontally aligned as shown in FIG. 14A when a voltage is applied.
The actual liquid crystal molecules m are not aligned completely vertically to the surface s of the display panel p, but as shown in FIG. 14C, they are slightly inclined from the vertical direction with respect to the surface s of the display panel p even when vertically aligned. Such a condition similarly occurs in the types of liquid crystal described in the prior art. At this time, even if incident light is vertically incident on the surface s as shown by the arrow A, a slight birefringent effect acts on the incident light. It is called xe2x80x9cremaining phase effectsxe2x80x9d that the birefringent effect is produced by the incident light being obliquely incident or by the liquid crystal molecules being slightly inclined from the vertical direction as described above.
In the ferroelectric liquid crystal, no such xe2x80x9cremaining phase effectsxe2x80x9d is caused in principle. Instead, because of the structure in which the birefringent effect always acts and the modulation is caused by changing the axial direction of birefringence, the phase difference error due to the thickness error of the ferroelectric liquid crystal itself is caused. Therefore, because of this error as well as the above-described wavelength dependence of the polarizing plate and the quarter-wave plate, it is also difficult to provide perfect black display with the ferroelectric liquid crystal.
Other examples of the prior art include one using a so-called scattering mode, and a guest-host type. However, high contrast is not obtained in principle in these examples. Another example is one using two polarizing plates, one in front of and the other behind the liquid crystal. However, this structure is inefficient.
An object of the present invention is to provide a display optical apparatus being simple in structure, excellent in efficiency and low in cost, with which high contrast is obtained, and a projector display apparatus using the display optical apparatus.
To achieve the above-mentioned object of the present invention, a display optical apparatus according to the present invention is provided with: a polarizing plate transmitting illuminating light having a predetermined polarization direction; a reflective liquid crystal display device reflecting the illuminating light transmitted by the polarizing plate as projected light of a polarization condition that differs according to pixel information for each pixel; and a phase plate changing the polarization conditions of the illuminating light and the projected light and directing the illuminating light and the projected light to the polarizing plate. The phase plate is disposed between the reflective liquid crystal display device and the polarizing plate. The black level of the projected light is adjusted by rotating the principal axis direction of the phase plate.
A projector display apparatus according to the present invention is provided with a projection optical system and a display optical apparatus. The display optical apparatus is provided with: a polarizing plate transmitting illuminating light having a predetermined polarization direction; a reflective liquid crystal display device reflecting the illuminating light transmitted by the polarizing plate as projected light of a polarization condition that differs according to pixel information for each pixel; and a phase plate changing the polarization conditions of the illuminating light and the projected light and directing the illuminating light and the projected light to the polarizing plate. The phase plate is disposed between the reflective liquid crystal display device and the polarizing plate. The black level of the projected light is adjusted by rotating a principal axis direction of the phase plate.